The present invention relates to pressure vessels, and more particularly to pressure vessels having oscillator and sensor means, and to a method and apparatus for determining if there may be deterioration in the structural integrity of such pressure vessel.
Due to the need of alternative fuelled vehicles, such as natural gas, propane, and potentially fuel-cell powered vehicles, to carry highly pressurized gaseous, or dual-phase (gaseous and liquid) fuels, pressure vessels containing such highly pressurized fuels for such vehicles must be structurally sound and be able to withstand such high pressures.
Up to the early 1990""s, pressurized fuel tanks for alternative or dual fuel (propane or natural gas) vehicles were typically cylindrical steel tanks, located in the trunk or in the traditional gasoline tank location for such vehicle. In order to save weight, such tanks were typically of a limited thickness, and thus could only be pressurized to a pressure in the range of 3000-3600 psi (20.68-24.82 MPa), and to conserve weight, they were of a limited size. The range of travel of the vehicle on a single tank fill was accordingly very limited due to not being able to carry more gaseous fuel, resulting in limited range of such vehicles on a single tank fill.
A need arose to have such pressure tanks able to withstand greater pressures, without greatly increasing the weight of such tank, so as to be able to carry more fuel without greatly increasing the tank size (and thus weight). This need has become more acute with the likely and coming introduction of fuel cell vehicles, which require substantial quantities of hydrogen which can only be stored in a gaseous pressurized form. Due to the lesser energy density of hydrogen as compared with natural gas and gasoline, even greater quantities are needed to power a vehicle for the same distance, and thus tanks able to withstand even greater pressures (so as to certain even greater quantities) of such gas are required.
Companies such as Dynetek, Inc. of Calgary, Alberta have developed specialized carbon-fiber composite pressure vessels, which are specially adapted for containing gaseous fuels at high pressure, in the range of 3000-10,000 psi (20.68-68.95 MPa). These specialized tanks typically consist of a substantially cylindrical inner vessel, typically of aluminum or a plastic. To the exterior surface of such inner vessel is wound a plurality of carbon fiber strands embedded in a polymer composite (CFRP), which is cured to form an exterior, extremely lightweight shell (known as a CFRP shell), highly resistant to tensile and hoop stresses to which it is subjected to by the compressed gases within the inner vessel. The exterior CFRP shell, which is typically comprised of a series of strands of carbon fiber which are wound about the exterior surface of the inner pressure vessel and held together with a polymer resin, effectively bears the bulk of the hoop stresses exerted by the highly pressurized gases which are injected into the inner pressure vessel.
These CFRP pressure vessel fuel storage tanks are adapted for storing propane, methane and/or natural gas, but are particularly adapted for storing hydrogen for fuel cell vehicles.
These CFRP pressure vessels, like all other pressure vessels, can become weakened through fatigue due to cyclic stresses which arise due to the high pressures involved and the continual filling, exhausting, and re-filling such tanks. In addition, the structural integrity of such CRFP pressure vessels may be compromised as a result of structural damage to the exterior CFRP shell due to cuts, gouges, or deformation thereto arising in the handling, storage, or filling of such pressure vessel. Alternatively, such pressure vessels can become damaged through overpressure, if for example, the pressure relief valve which is typically installed on such tanks was to fail or become inoperative.
Importantly, due to the extremely flammable nature of stored gaseous fuels such as natural gas and hydrogen, and due to the extremely high pressures under which such gas is stored, it becomes of paramount importance that such pressure vessels be structurally sound. Otherwise, due to the intended application of such tanks for use in motor vehicles or bulk transportation of gases on public highways, structural damage of such tanks can result in catastrophic failure of such tanks, and ignition of the flammable contents upon release of such contents to the atmosphere, with likely resultant loss of human life as well as material and property damage. Accordingly, it is of paramount importance that damaged or structurally compromised tanks be immediately removed from service.
Unfortunately, it is not easy, and in most cases impossible to determine if the structural integrity of a CFRP pressure vessel has been compromised from a simple visual inspection of such pressure vessel.
Accordingly, a real and clear need exists for a method to determine if the structural integrity of a modern CFRP pressure vessels has been possibly compromised.
A further need exists for a pressure vessel which is able to self-monitor and warn when structural integrity thereof may have been compromised.
Lastly, a further need exists for an apparatus to be able to determine if the structural integrity of a pressure vessel has been compromised.
Fulfillment of these needs allows an important advance in the implementation of CFRP pressure vessels, and allows structurally-compromised pressure vessels to be withdrawn from use and thereby reduce i) the possibility of leakage from such pressure vessels and the consequent loss of such fuel, or, worse yet ii) catastrophic failure resulting in explosion due to the extremely high pressures to which the vessel is subject to.
U.S. Pat. No. 5,522,428 teaches a composite pressure vessel 20, having a three types of sensors, namely a pair of strain sensors 46, a pair of temperature sensors 48, and a pair of acoustical sensors 44, all of which are applied to the exterior of the load-bearing composite shell 28 of the composite vessel (ref. col. 3, lines 5-12). All sensors are connected to a microprocessor (CPU) 32, which is in turn connected to a solenoid valve 26, which controls flow of gas to/from the vessel. The strain sensors 46 permit the microprocessor to count the number of fill cycles to which the vessel is subject, to keep track of when the cyclic stresses on the vessel may be reaching the design limit thereof. The temperature sensors 48 allow over-temperature conditions (and thus possible structural degradation to the vessel) to be sensed. The acoustic emission sensors 44 count the sound emissions in a given time period above a trigger level intensity, which if above a certain level, may indicate imminent failure. If any of certain conditions are sensed, the microprocessor 32 may cause the solenoid 26 to prevent refill of the pressure vessel 20.
The aforesaid method disclosed in US ""428 for monitoring pressure vessel integrity involves numerous types of different types of sensors, and as such does not provide a single sensor capable of determining in and by itself the tank integrity.
Moreover, acoustical sensors are relatively large in size, and are relatively expensive. Furthermore, in motor vehicle (ie. xe2x80x9cnoisyxe2x80x9d) environments, acoustic sensors may be unreliable. Lastly, there is no teaching in such patent as to how the xe2x80x9ccountxe2x80x9d of pressure cycles is to be maintained by the microprocessor in the event of interruption of electrical power to the microprocessor. Indeed, it seems clear from this patent that electrical power must continue to be supplied to the microprocessor (col. 5, line 15-16xe2x80x94xe2x80x9cwith current battery technology, it is likely that the CPU 32 can be maintained by an integral batteryxe2x80x9d).
Accordingly, a real need continues to exist for a lower cost method and apparatus for being able to determine pressure vessel structural integrity.
The present invention makes use of the concept that a structural change in a component (which may indicate a deterioration in the structural properties of the component) results in a change in the value of the various natural frequency resonances of such component, and more particularly, results a shift in the frequencies at which the natural resonances occur and/or a decrease or increase in the amplitude and shapes one or more of the natural frequency resonances.
Accordingly, changes in the value, amplitude, and/or shape of the natural frequency response of a component when new, as compared to the value, amplitude, and/or shape of the natural frequency response at a later time At after such component has, for example, been subject to a number of stress cycles, may possibly indicate a change in the physical properties, including a reduction in the structural integrity of such component.
Accordingly, the present invention, in one of its broad embodiments, provides a method for determining if there may be deterioration in structural integrity of a pressure vessel having an exterior surface, where both sensor means and oscillating means are affixed to or embedded in such exterior surface. Such method comprises the steps of:
(a) providing an electrical current to said oscillator means so as to cause said oscillator means to oscillate so as to produce a mechanical disturbance to said exterior surface and cause a detectable natural frequency response of said pressure vessel;
(b) detecting, using said sensor means, said natural frequency response;
(c) recording said natural frequency response;
(d) after a period of elapsed time, repeating steps a) and b) above; and
(e) comparing said recorded natural frequency response first obtained with said resultant natural frequency response last obtained, and determining if there be a difference therebetween.
Notably, depending on the type of composite shell used for the pressure vessel, the natural frequency profile of the pressure vessel may not substantially change as a result of minor changes in temperature and/or pressure to which the pressure vessel, and in particular the composite load-bearing shell thereof, may be subjected to. Accordingly, the method of the present invention is particularly suited to CFRP pressure vessels having an inner vessel and a composite (load-bearing) exterior shell, since the natural resonant response of such load-bearing CFRP shell is not particularly affected by changes in temperature and/or pressure,
However, in instances where the natural frequency profile of the pressure vessel is sensitive to changes in pressure and/or temperature to which the vessel is subject, or alternatively, in instances where the natural frequency profile last obtained is sensed at a temperature and/or pressure substantially different than what the pressure vessel was subject to initially so that the natural frequency response last obtained differs with that first obtained due to these changes in temperature and/or pressure, the method of the invention includes various refinements.
In a first refinement, wherein said mechanical disturbance in step a) occurs when said pressure vessel is subject to no internal pressure or a selected internal pressure, said steps a) and b) are repeated as per step d) at a time when said pressure vessel is subjected to said same no pressure or said selected pressure in order to be able to properly compare the natural frequency profile last obtained with the natural frequency profile first obtained.
In an alternative (or additional) refinement, wherein said mechanical disturbance in step a) occurs when said pressure vessel is subject to a particular temperature, said steps a) and b) are repeated as per step d) at a time when said pressure vessel is subjected to said same particular temperature.
Finally, in a further embodiment of the invention where the natural frequency response of the pressure vessel is sensitive to changes in pressure and/or temperature, or where it is desired to obtain the last-obtained natural frequency response at a time when the temperature and/or pressure to which the pressure vessel is subject is substantially different than when the natural frequency response was first obtained, the method of the present invention contemplates applying a correction to the natural frequency profile last obtained to compensate for such change in temperature and/or pressure, in order to be able to properly compare such response with the natural frequency response first obtained.
Accordingly, in a method of the present invention where mechanical disturbance in step a) occurs when said pressure vessel is subject to a particular pressure and/or temperature, and step a) and b) are repeated as per step d) when said pressure vessel is subjected to a different pressure and/or temperature, then in a further embodiment of the present method such method comprises, prior to step e), applying a correction to adjust said last-obtained natural frequency response for said different pressure and/or temperature, so as to be able to compare said frequency response first obtained with said frequency response last obtained.
The sensor means, in a preferred embodiment of the present invention, comprises a piezo-electric material affixed to the surface of the pressure vessel, or embedded in the fiber-reinforced shell of such CFRP pressure vessel. Sensors of this type have been found to be suitable when so placed to detect natural frequency responses of such pressure vessel.
Similarly, oscillation means in the form of a piezo-electric material affixed to the surface of the pressure vessel, or embedded in the fiber-reinforced polymer shell of such pressure vessel so as to protect such piezo-electric material from exposure to the elements and/or damage, has been found to be suitable when so placed for generating a mechanical disturbance in such polymer shell sufficient to generate a detectable natural frequency response of such pressure vessel.
In a further embodiment of the method of the present invention, said step of comparing the natural frequency response first obtained with the resultant natural frequency response last obtained comprises obtaining a voltage response from the sensor means as a function of time, and comparing said voltage response with a later obtained voltage response as a function of time, and determining if there be any differences, including such differences as to any shift in the value of the natural frequencies of such component, the amplitude of the natural frequencies, and/or the shape and/or amplitude of the natural frequency response of such component. In yet a futher refinement of the method as set out above, the step of comparing said resultant natural frequency response first obtained with said natural frequency response last obtained comprises:
i) calculating a power spectrum density as a function of frequency from said resultant natural frequency response of said piezo-electric sensor means first obtained, and
ii) calculating a power spectrum density as a function of frequency from said natural frequency response of said piezo-electric sensor means last obtained; and
iii) comparing said power spectrum density obtained from step ii) above with that obtained from step i) above, and determining if there are differences.
In another aspect of the present invention the invention includes a structural integrity testing apparatus for a pressure vessel having an exterior surface comprised of a composite material having both sensor means and oscillator means affixed to or embedded in said exterior surface; comprising:
means for applying an electrical currrent to said oscillator means so as to cause said oscillator means to oscillate so as to produce a mechanical disturbance to said exterior surface and cause a detectable natural frequency response of said pressure vessel;
means for recording said natural frequency response of said pressure vessel; and
means for comparing said recorded resultant natural frequency response with a later-obtained natural frequency response, and determining if there be any difference therebetween.
In a further refinement of the test apparatus of the present invention, such apparatus further comprises means for indicating lack of pressure vessel integrity to an operator if said means for comparing indicates differences were detected.
In yet a further refinement of the test apparatus of the present invention, such apparatus further comprises means for applying a correction to said later-obtained natural frequency response to adjust for any differences in temperature and/or pressure to which said pressure vessel may be then subject as compared to temperature and/or pressures which it was subject at the time of first recording said resultant natural frequency response.
In a still further aspect of the present invention, the present invention relates to a pressure vessel. Such pressure vessel comprises:
an inner vessel;
a composite material surrounding said inner vessel and forming an exterior surface;
both sensor means and oscillator means affixed to or embedded in said exterior surface; and,
means for storing a natural frequency response as received from said sensor means, after a mechanical disturbance has been provided to said exterior surface of said pressure vessel by said oscillator means.